UNIT IV BRAKES Elementary theory of the shoe brake
Consider the simple shoe shown in Fig. 37.7. An actuating force W will give rise to a normal force P between the shoe and the drum (this force is shown as it acts ac ts on the shoe) and this normal force will produce a frictional force P if the drum is rotating as shown. !ow the shoe is in e"uilibrium under the action of the forces shown# together with the forces acting at the pivot# but the latter have no moment about the pivot and conse"uentl$ the cloc%wise moments due to the forces P and P must be balanced b$ the anti&cloc%wise moment due to W. 'ence we get the relation !ow the bra%ing tor"ue acting on the drum is due entirel$ to the frictional force P and is e"ual to P * or# substituting the epression obtained above for P# wet get +ra%e tor"ue# ,t Considering the show shown in Fig. 37.# 37. # the balance of moments about the pivot gives
so that the epression for the bra%ing tor"ue is iW/* 0t is now easil$ seen that ,1 is greater than ,t# the other factors being e"ual. /et i - 2.# / - 2.14 m# 5 - 2.274 m# * - 2.1 m and W - 422 !. ,hen , - 2. 422 2.14 2.1 - 3 - 6 !m , 2.274 & 2.2 2.234 ,hus ,1 is 3.3 times ,t. ,he shoe shown in Fig. 37.7 is called a trailing shoe while that shown in Fig. 37. is called a leading shoe. 0t should be clear# however# that in a conventional bra%e the leading shoe will become the trailing one if the direction of rotation of the bra%e drum is reversed# and vice versa. An actual bra%e shoe acts in a similar manner to the simple one considered above# the onl$ difference being that the frictional force iP will act at a larger radius than the radius of the bra%e drum and this will accentuate the difference between the tor"ues developed b$ the shoes. 0n a bra%e of the t$pe shown in Fig. 37.3# however# the epanding cam will not appl$ e"ual forces to the shoes but will appl$ a greater force to the trailing shoe. ,a%ing the data assumed
above and supposing that a total actuating force of 1222 ! is available then the cam would appl$ a force of 767 ! to the trailing shoe and onl$ 33 ! to the leading shoe. ,he total bra%ing tor"ue would then be 222 !m. 0f# however# the whole 1222 ! available for actuation had been applied to the leading shoe alone then the bra%e tor"ue would have been 17 1 !m# that is# more than twice as great and this result can be obtained b$ ma%ing both shoes leading shoes and appl$ing 422 ! to each. 0f the actuating mechanism were of the t$pe that applies e"ual forces to the shoes then each actuating force would be 422 ! and the total bra%e tor"ue developed b$ the shoes would be 471 8 62 - 11 179 !m. ,hus a floating or e"ualising actuating mechanism gives an increase in bra%e tor"ue for a given actuating force but it has the disadvantage that the wear of the leading shoe (assuming the shoes to have linings of the same material) would be 3.9 times that of the trailing shoe. ,he bra%e with two leading shoes would not suffer from this drawbac% and# as has been seen# gives an even greater bra%e tor"ue. :uch bra%es are# therefore# widel$ used# particularl$ for front wheels. When h$draulic actuation is used it is a simple matter to ma%e both shoes leading ones for the forward direction of rotation; the bra%e is arranged as shown in Fig. 37.9# two actuating c$linders# connected b$ a pipe# being used instead of one c$linder. For the reverse direction of rotation both shoes would be trailing shoes and the bra%e would be rather wea%. For this reason it is usual to emplo$ the two&leading&shoe bra%e in the front wheels onl$# the rear bra%es being the conventional leading and trailing shoe t$pe.
When the bra%e actuation is mechanical it is not so simple to ma%e both shoes leading ones but a relativel$ simple mechanism has been developed b$
bear on the anchorage at the bottom# being once again a leading shoe. ,hus# b$ emplo$ing two shoes# each with the bell&cran% and strut mechanism# a bra%e which is a two&leading shoe bra%e for either direction of rotation or which re"uires onl$ one actuating mechanism is obtained.
Brake Shoe Energization ,he primar$ function of the bra%e drum assembl$ is to force the bra%e shoes against the rotating drum to provide the bra%ing action. When the bra%e shoes are forced against the rotating drum. the$ are pulled awa$ from their pivot point b$ friction. ,his movement# called self&energi=ing action (fig. 7&1)# draws the shoes tighter against the drum.
As the bra%e actuating mechanism forces the bra%e shoes outward# the top of the bra%e shoe tends to stic% or wedge to the rotating bra%e drum and rotates with it. ,his effect on bra%e shoes greatl$ reduces the amount of effort re"uired to stop or slow down the vehicle. With most drum bra%e designs# shoe energi=ation is supplemented b$ servo action. When two bra%e shoes are lin%ed together# as shown in figure 7&1# the application of the bra%es will produce a self&energi=ing effect and also a servo effect. :ervo action is a result of the primar$ (front) shoe attempting to rotate with the bra%e drum. +ecause of the fact that both shoes are lin%ed together# the rotating force of the primar$ shoe applies the secondar $# (rear) shoe. 0n the forward position# the anchor point for both bra%e shoes is at the heel of the secondar$# shoe. As the vehicle changes direction from forward to reverse# the toe of the primar$ shoe becomes the anchor point# and the direction of self&energi=ation and servo action changes (fig. 7& 1). ,he most popular bra%e drum configurations (fig. 7&14) are as follows> :ingle anchor# self&energi=ing servo action (fig. 7&14)& 0n this configuration both bra%e shoes are self&energi=ing in both forward and reverse directions. ,he bra%e shoes are self¢ering and provide servo action during bra%e application. ,his s$stem is provided with one anchor pin. which is rigidl$ mounted to the bac%ing plate and is nonad?ustable. +oth forward and reverse tor"ue is transmitted to the bac%ing plate through the anchor pin. @ne wheel c$linder with dual pistons is used in this s$stem. :ingle anchor# self¢ering (fig. 7&14)& 0n this configuration onl$ the primar$ bra%e shoe is self&energi=ing in the forward direction; therefore. it provides the ma?orit$ of the bra%e force. ,his s$stem is self¢ering in that the lower bra%e shoe anchor does not fi the position of the bra%e shoes in relation to the drum. ,he shoes are allowed to move up and down as needed. :ome s$stems provide eccentric cams for front and rear bra%e shoe ad?ustments. @ne wheel c$linder is provided in this s$stem. ouble anchor# single c$linder (fig. 7&14)& 0n this arrangement# each bra%e shoe is anchored at the bottom b$ rotating eccentric&shaped anchor pins. @nl$ the primar$ shoe is self&energi=ing. and the s$stem does not develop servo action. :pring clips are used in the middle of the shoe to hold the shoes against the bac%ing plate. +ra%e shoes are ad?usted manuall$ b$ rotating the anchor pins. @ne wheel c$linder is provided in this arrangement.
ouble anchor. double c$linder (fig. 7&14)& 0n this s$stem the bra%e shoes are provided with an anchor at each heel. ,he anchors are eccentric&shaped to allow for ad?ustment and centering. Bach shoe has a single piston wheel c$linder mounted at the toe of the bra%e shoe. Which allows both shoes to be self&energi=ing in the forward direction onl$. Bccentrics mounted in the middle of the shoe also allow for bra%e ad?ustment.
Disadvantages of Drum Brakes ,he drum bra%e assembl$# although well suited for wheeled vehicles# has some disadvantages. @ne
Figure 7&1.& :elf&energi=ing and servo action.
problem that occurs during heav$ bra%ing is bra%e fade. uring panic stops or repeated harsh stops# the bra%e linings and drum develop large amounts of heat that reduces the amount of friction between the bra%e shoe and drum. ,his reduction in friction greatl$ decreases the stopping abilit$ of the vehicle# and# in most cases# additional pressure directed on the bra%e pedal would not increase the stopping performance of the vehicle. ,he enclosed design of the bra%e drum assembl$ does not allow for cooling air to enter the assembl$ and therefore heat developed during bra%ing must be dissipated through the bra%e drum and bac%ing plate. As the bra%es heat up due to repeated application# cooling air flowing past the drums and bac%ing plates is limited. ,his condition causes the radius of the drum to increase more than the radius of the bra%e shoe. As a result# a change in pressure distribution between the linings and the drum occurs# which reduces the bra%ing abilit$ of a vehicle b$ up to 2 percent. ,he enclosed design also does not allow for water to be epelled rapidl$ should the bra%e cavit$ become wet due to adverse weather conditions. ,he water reduces the frictional properties of the bra%e s$stem and must be removed to restore bra%ing abilit$. ,his is a ver$ dangerous situation and drasticall$ reduces the stopping abilit$ of the ve hicle until the s$stem is dr$. ,he use of man$ clips and springs ma%es overhaul of the bra%e drum assembl$ ver$ time& consuming. +ecause of the enclosed drum. asbestos dust is collected in the bra%e cavit$ and certain parts of the bra%e drum. CAUTIO
Asbestos can cause cancer.
!a"uum servo assisted brake ,he term Dvacuum servoD is actuall$ a generic term for an$ device which uses a vacuum to amplif$Eboost the mechanical effort of a device b$ use of a vacuum in an assisting chamber# either attached to the input or output# or placed between the input and output. 0n a acuum bra%e servo# the unit is placed between the bra%e pedal and the h$draulic master c$linder# using stored vacuum to amplif$ the drivers pedal efforts# giving a greater bra% ing force. ,he term Dvacuum :ervoD can sometimes be mis&translated in vehicle engineering terms# to Dvacuum operated valveD whereb$ a device is controlled either switched or proportionall$ b$ the use of a acuum. ,his device can be readil$ seen in modern off road vehicles to engage W and differential loc%s# and in older vehicles fitted with vacuum controlled servo motor cruise control.
A bra%e booster is used on virtuall$ all vehicles which use h$draulic bra%es for their primar$ bra%ing circuit. acuum servos are not used on vehicles which use cables# rods (or other mechanical lin%ages)# or pressuri=ed air s$stems for their primar$ bra%e circuits. A acuum :ervo also %nown as a power booster or power bra%e unit uses a vacuum to multipl$ the drivers pedal effort and appl$ that effort to the master c$linder .1G ,he vacuum can be generated in two distinct methods# dependent on the t$pe of internal combustion engine# or other motive force (as in electric vehicles). 0n naturall$&aspirated petrol engines#
the manifold vacuum is
used# whereas in turbo charged# diesel engines# and
electricEh$brid vehicles a separate vacuum pump is used (or in certain high altitude places# naturall$&aspirated vehicles are not capable of producing enough vacuum for the booster. ,he vacuum pump can be driven mechanicall$ (from the engine) or b$ means of an electric motor. ,he vacuum is transferred to the servo along non&collapsible vacuum lines# and is stored in the servo b$ using a non&return valve. ,he vacuum booster or vacuum servo is used in most modern h$draulic bra%e s$stems which contain four wheels.citation needed G ,he vacuum booster is attached between the master c$linder and the bra%e pedal and assists the bra%ing force applied b$ the driver. ,hese units consist of a hollow housing with a movable rubber diaphragm across the center# creating two chambers. When attached to the inta%e manifold of the engine or the vacuum pump# the pressure in both chambers of the unit is lowered. ,he e"uilibrium created b$ the low pressure in both chambers %eeps the diaphragm from moving until the bra%e pedal is depressed. A return spring %eeps the diaphragm in the starting position until the bra%e peda l is applied. When the bra%e pedal is applied# the movement pushes against a small spring which pushes against an air valve thus opening it to allow atmospheric pressure air to flow into the Dsuppl$D chamber of the booster. @nce the pedal stops advancing forward# the air valve closes again and can open further to allow more air in for continued boosted bra%ing. :ince the pressure becomes higher in one chamber (or Dsuppl$D chamber)# the diaphragm moves toward the lower pressure chamber (or DvacuumD chamber) with a force created b$ the area of the diaphragm and the differential pressure. ,his force# in addition to the driverHs foot force# pushes on the master c$linder piston. When the pedal is allowed to return to rest# the air from the suppl$ chamber escapes to the vacuum chamber and can then flow towards the source of vacuum. A relativel$ small diameter booster unit is re"uired; for a ver$ conservative 42I manifold vacuum# an assisting force of about 1422 ! (142 %gf) is produced b$ a 2 cm radius diaphragm with an area of 2.23 s"uare meters. ,he diaphragm will stop moving when the forces on both sides of the chamber reach e"uilibrium. ,his can be caused b$ either the air valve closing (due to
the pedal appl$ stopping) or if Drun outD is reached. *un out occurs when the pressure in one chamber reaches atmospheric pressure and no additional force can be generated b$ the now stagnant differential pressure. After the run out point is reached# onl$ the driverHs foot force can be used to further appl$ the master c$linder piston. 0n some new models# in addition to providing functionalit$ for A+:# ,raction Control# and d$namic stabilit$ control# bra%e h$draulics are being used to augment bra%ing when little or no vacuum is detected in the booster# or booster run&out is reached# or pre&appl$ing the bra%es if certain panic bra%ing or wet driving conditions are detected. Brake booster
A bra%e booster is an enhanced master c$linder setup used to reduce the amount of pedal pressure needed for bra%ing. 0t emplo$s a booster set up to act with the master c$linder to give higher h$draulic pressure to the bra%es andEor lower force applied on the bra%e pedal through a bra%e booster push&rod. ,he bra%e booster usuall$ uses vacuum from the engine inta%e to boost the force applied b$ the pedal on to the master c$linder# or ma$ emplo$ an etra vacuum pump to enable it. Without the engine running the bra%e pedal feels ver$ hard and ineffective on the bra%ing capabilit$. An DactiveD booster is a non DconventionalD booster where a solenoid is used to open the booster air valve to automaticall$ push the master c$linder forward to perform some forms of d$namic stabilit$ control. +ra%e boosters come in either a single diaphragm or tandem diaphragm (which is generall$ used for bigger vehicles and truc%s). ,he$ can be Dcabin& breathersD (ta%ing clean filtered air from inside the cabin thus ma$ be more nois$) or Dengine& breathersD (less nois$ but more at ris% for becoming clogged with mudEice if not protected properl$). Apart from this additional booster setup# the bra%ing s$stem is a normal h $draulic bra%e s$stem. Brake servo
,he bra%e servo supports the force the driver eerts on the master bra%e c$linder when heEshe presses the bra%e pedal. ,his significantl$ reduces the effort re"uired when bra%ing. ,ogether with the master bra%e c$linder... Function ,he bra%e servo supports the force the driver eerts on the master bra%e c$linder when heEshe presses the bra%e pedal. ,his significantl$ reduces the effort re"uired when bra%ing. ,ogether with the master bra%e c$linder# it is a component of most bra%ing s$stems in cars. ,he two most common designs are
acuum bra%e servo> 5ost bra%e s$stems in cars have a vacuum bra%e servo. ,he$ use the vacuum that is produced in petrol engines b$ the air inta%e s$stem in the engineHs inta%e pipe or via a vacuum pump (2.4...2.9 bar) in diesel engines. '$draulic bra%e servo> ,hese t$pes of servo use the pressure created b$ a h$draulic pump# which is driven b$ the engine. '$draulic bra%e servos are used in vehicles which have h$draulic energ$ suppl$ (e.g. power steering) and vehicles whose engine has low vacuum pressure in the inta%e pipe (e.g. turbo engines). '$draulic bra%e servos are smaller than vacuum bra%e servos and re"uire a higher pilot pressure. ,here is a membrane inside the bra%e servo# which divides the servo into two chambers. When the bra%e is not being operated# there is a vacuum in both chambers# which is generated b$ the engine. When the bra%e is operated# both of these chambers are sealed off from one another. At the same time# a valve opens which allows atmospheric pressure to flow in on the pedal side. !ow there is atmospheric pressure on one side of the membrane (pedal side) and on the other side a vacuum (master c$linder side)# which pulls the membrane connected to the push rod towards the master c$linder and augmenting the force from the pedal. When the bra%e pedal is released# both chambers are reconnected with each other via a valve opening whilst the valve# which was previousl$ allowing atmospheric pressure to flow in# now closes. ,here is now a vacuum in both chambers. ,he bra%e servo onl$ wor%s when the engine is running. 0f the engine is switched off# e.g. when the vehicle is being towed# the bra%e force must be applied solel$ via the pedal. Safety ,he bra%e is a safet$&relevant part of the vehicle. A functioning bra%e servo helps the driver when bra%ing. 0f it fails# the driver must then appl$ much more pressure to the bra%e pedal than heEshe is used to when the bra%e servo is functioning. ,herefore# should the driver notice a drop in bra%ing effect# the vehicle should alwa$s be ta%en to a specialist garage. ,esting the functionalit$ of the bra%e servo in four steps 0t is generall$ possible to test if the bra%e servo is wor%ing properl$ using the following measures> 1. ,urn off the engine . Press the bra%e pedal repeatedl$ until $ou feel a strong resistance J(this means the vacuum still present in the s$stem has been Dused upD) 3. Keep the bra%e pedal pressed down . :tart the engine. 0f the bra%e pedal now $ields# the servo is wor%ing properl$. ,his test is however no substitute for visiting a specialist garage as soon as the driver notices that the bra%e effect is droppingL
Depreciation +ra%es that function reliabl$ are vital for safe travel in motor vehicles. ,he$ should therefore be chec%ed regularl$ to ensure that the$ are in perfect wor%ing order and that their component parts are not suffering from wear. A well&maintained bra%e that is in perfect wor%ing order which a driver feels can be relied upon is the most important step on the wa$ to feeling safe when driving.
Servo Operation 0f the force applied comfortabl$ b$ a driver to the footbra%e is insufficient to retard the vehicle at the re"uired rate# some form of assistance is necessar$. ,he boosting force applied to supplement the driverMs effort is called servo assistance. 0n the past servo assistance was provided b$ rotation of the bra%e drum (self&servo) to %eep the pedal force low. ,oda$ due to introduction of powerful disc bra%es# the servo assistance is provided b$ either pneumatic or h$draulic means. 0n practice# vacuum assistance is added for medium cars# h$draulic assistance for heav$ cars and vehicles fitted with anti&loc% bra%ing s$stems# and compressed&air assistance for some light truc%s and minibuses.
Vacuum Assisted Servo ,his servo s$stem is the most popular. ,he induction manifold depression of the spar%&ignition engine is used as source of servo energ$ in most s$stems. :ince vacuum energ$ is not available at the manifold of a diesel engine# an engine&driven NvacuumM pump (ehauster) in this case provides the re"uired assistance. All servo s$stems must be fail&safe# i.e. if a fault develops in the vacuum unit the main bra%ing s$stem must be operative# however with a peda l force much greater than normal. ,he opera tion of the servo should be progressive# i.e. the servo assistance should be proportional to the ped al effort for light pedal pressure. Figure .42 represents the relationship between the h $draulic pressure acting on the bra%e c$linders and the pedal effort for both with and without servo assistance. ,he servo valves provide a progressive assistance up to the %nee point where the maimum vacuum assistance is received and an$ rise in output pressure be$ond this point is onl$ due to the increased pedal effort. acuum servos in use toda$ are called suspended&vacuum s$stems# because Nvacuum conditionsM prevail on both sides of the servo piston during operation of the vehicle with the bra%es off. When the bra%e is applied# outside air is bled in to the chamber on one side of the piston to create a pressure difference. ,his arrangement allows the servo to respond "uic%l$ in com parison to the
older atmospheric suspended t$pe s$stem. 0n this older s$stem# air is present on both sides of the piston and the air is Ndrawn outM to provide assistance. ,he two main t$pes of suspended&vacuum servos are the indirect and direct t$pe.
#ig$ %&$'($ Servo assistan"e$
How a vacuum brake booster works
A vacuum bra%e booster provides power assist to the bra%e s$stem. As an$one who has eperience a booster failure %nows# the booster greatl$ reduces the effort re"uired to stop the vehicle. +ra%e booster problems misdiagnosed# often set off a series of epensive events. acuum bra%e boosters ma$ be m$sterious to the average person# but are "uite simple in operation. ,he vacuum operated bra%e booster wor%s much toda$ as it did sit$&$ears ago. A combination of atmospheric pressure pushing and vacuum pulling on a diaphragm# multiplies the force a driver applies with the bra%e pedal. 5ost bra%e boosters# have two or more chambers# divided b$ fleible diaphragms. ,he chamber is normall$ steel and the diaphragm is usuall$ some form of rubber. ,he$ attach a metal push rod to the diaphragms and it moves with them. acuum# usuall$ supplied b$ the engine and atmospheric pressure combine to provide the assistance during bra%ing.
,he booster at rest
When we are not appl$ing the bra%es# a two&wa$ valve allows vacuum application on both sides of the diaphragm. An e"ual vacuum on both sides# causes a balance# and the diaphragm remains stationer$. 0n this state we sa$ that the booster is at rest. ,he shell of the booster acts as a reservoir# to store the engine vacuum. A chec% valve# in the inlet fitting# helps to maintain a vacuum when the engine accelerates.
Appl$ing the bra%es
When we press the bra%e pedal# the two&wa$ valve also moves. ,his movement closes the passage to the rear of the diaphragm and vacuum applies onl$ to the front. :imultaneousl$ atmospheric pressure flows into the rear. Atmospheric pressure pushes the diaphragm and vacuum pulls it forward. ,he push rod also moves to appl$ the bra%es# through the master c$linder # attached to the front of the booster. *eleasing the bra%e pedal allow the internal spring to push the diaphragm and push rod bac% and operates the two&wa$ valve. ,his valve bloc%s atmospheric pressure to the rear chamber. :imultaneousl$# it opens the chamber to vacuum. ,his evacuates the rear chamber and assist in returning the booster to a state of rest.
,he s$stem is robust and inherentl$ failure resistant. 0f the valve# diaphragm or vacuum source fail# the$ have mechanicall$ connected the push rod to the bra%e pedal and master c$linder. A damaged s$stem reverts to manual bra%es# with no power assist. ,$pes of failures 5ost common booster failures are# a lac% of assistance# noise or poor pedal return after release. @ften a good booster is mis&diagnosed as bad. !eedlessl$ replacing a good booster often occurs.
,o function properl$# the bra%e booster needs a good source of a vacuum. ,he lac% of a vacuum is more common than bra%e booster failure# when we have no power assist. Collapsed and crac%ed hoses are most common. @ther issues include a plugged vacuum port in the inta%e and a bad chec% valve in the inlet. +efore replacing a bra%e booster# we must alwa$s test the vacuum source at the bra%e booster. We do this with a vacuum tee and a vacuum gauge.
@ther bra%e booster failures include bro%en springs# ruptured diaphragms and lea%ing valves. When the diaphragm ruptures# atmospheric pressure lea%s through and does not provide power assistance. A bro%en spring most often causes the bra%e pedal to not return full$ when released. +ra%e lights sta$ing on# are one possible s$mptom# and possibl$ overheated bra%es.
A ruptured diaphragm also allows# atmospheric pressure to enter the inta%e manifold. ,his creates a vacuum lea% and causes the fuelEair miture to lean out. @ther than hard bra%ing# another s$mptom is an engine misfire at idle# when we depress the bra%e pedal. Atmospheric pressure lowers the engine vacuum# b$ lea%ing through the diaphragm. 0t is simple to test for this. We can unplug the hose at the bra%e booster and bloc% the end with our thumb. 0f temporaril$ unplugging the booster and bloc%ing the hose solves the misfire# it shows a lea%ing booster. 0nternal valve failure ma$ result in a lac% of bra%ing assist or bra%es that do not release# depending on the failure. We most often replace boosters as an assembl$ as repair in the field is not practical. ,esting the booster We use a vacuum gauge to chec% for an ade"uate vacuum. ,he vacuum at the bra%e booster should be around 16&2 inches. 0nsufficient amounts can result from a restricted source and sometimes engine running problems. For instance a pluggedcatal$tic converter can lower engine vacuum and result in poor bra%e booster performance. A bad vacuum lea% in the inta%e ma$ also cause a lower vacuum. A lea%ing bra%e booster ma$ also cause an engine to run badl$. /ea%s in the bra%e booster provide a vacuum lea% to the engine. @ne "uic% test for lea%age# is to turn the engine off and press the bra%e pedal. 0f the pedal still has one or two assisted applications before getting hard to press# li%el$ no lea% eists. Another simple test is to appl$ the bra%e pedal several times without the engine running# to ehaust the vacuum. After the pedal becomes hard to push# hold it down and start the engine. A good booster# with an ade"uate vacuum will cause the pedal to drop slightl$. ,his is because the booster greatl$ increases pressure applied b$ the pedal.
:ome reasons a vacuum booster fails
When bra%e boosters fail the reason is often outside the booster itself. For instance a bad master c$linder lea%s fluid into the booster. +ra%e fluid will cause deterioration of the diaphragm resulting in failure. 0f misdiagnosed# the failed booster can provide a vacuum to the rear of a replacement master c$linder and "uic%l$ ruin it# repeating the c$cle. Close inspection of both the booster and the master c$linder is necessar$# when a failure occurs. As with most things# careful diagnosis can prevent an epensive rewor%. Power bra%es are no problem# for the professionals at A
REGENRATIVE BRAKING
,he most common form of regenerative bra%e involves an electric motor as an electric generator. 0n electric railwa$s the electricit$ generated is fed bac% into the suppl$ s$stem. 0n batter$ electric and h$brid electric vehicles# the energ$ is stored chemicall$ in a batter$# electricall$ in a ban% of capacitors# or mechanicall$ in a rotating fl$wheel. '$draulic h$brid vehicles use h$draulic motors to store energ$ in the form of compressed air . )ra"ti"al regenerative braking editG
*egenerative bra%ing is not b$ itself sufficient as the sole means of safel$ bringing a vehicle to a standstill# or slowing it as re"uired so it must be used in con?unction with another bra%ing s$stem such as friction&based bra%ing. •
,he regenerative bra%ing effect drops off at lower speeds# and cannot bring a vehicle to a complete halt reasonabl$ "uic%l$.
•
A regenerative bra%e does not immobilise a stationar$ vehicle; ph$sical loc%ing is re"uired# for eample to prevent vehicles from rolling down hills.
•
5an$ road vehicles with regenerative bra%ing do not have drive motors on all wheels (as in a two&wheel drive car); regenerative bra%ing is normall$ onl$ applicable to wheels with motors. For safet$# the abilit$ to bra%e all wheels is re"uired.
•
,he regenerative bra%ing effect available is limited# and insufficient in man$ cases# particularl$ in emergenc$ situations.
•
,he friction bra%e is a necessar$ bac%&up in the event of failure of the regenerative bra%e.
*egenerative and friction bra%ing must both be used# creating the need to control them to produce the re"uired total bra%ing. ,he <5 B&1 was the first commercial car to do this. 0n 1997 and 199 engineers Abraham Farag and /oren 5a?ersi% were issued two patents for this brakeby-wire technolog$.1GG Barl$ applications commonl$ suffered from a serious safet$ ha=ard> in man$ earl$ electric vehicles with regenerative bra%ing# the same controller positions were used to appl$ power and to appl$ the regenerative bra%e# with the functions being swapped b$ a separate manual switch. ,his led to a number of serious accidents when drivers accidentall$ accelerated when intending to bra%e# such as the runawa$ train accident in WOdenswil# :wit=erland in 19# which %illed twent$&one people.
Conversion to electric energy: the motor as a generator[edit
A ,esla 5odel : P48 using regenerative bra%ing power in ecess of 62 %W. uring regenerative bra%ing the power indicator is green Blectric motors# when used in reverse function as generators# convert mechanical energ$ into electrical energ$. ehicles propelled b$ electric motors use them as generators when using regenerative bra%ing# bra%ing b$ transferring mechanical energ$ from the wheels to an electrical load. Barl$
eamples
of
this
s$stem
were
the front&wheel
drive conversions
of
horse&
drawncabs b$ /ouis Antoine Krieger in Paris in the 192s. ,he Krieger electric landaulethad a drive motor in each front wheel with a second set of parallel windings ( bifilar coil) for regenerative bra%ing.3G 0n Bngland# the *aworth s$stem of Dregenerative controlD was introduced b$ tramwa$ operators in the earl$ 1922s# since it offered them economic and operational benefits as eplained b$ A. *aworth of /eeds in some detail.G4G6G ,hese included tramwa$ s$stems at evonport (1923)#*awtenstall# +irmingham# Cr$stal Palace&Cro$don (1926)# and man$ others. :lowing the speed of the cars or %eeping it in control on descending gradients# the motors wor%ed as generators and bra%ed the vehicles. ,he tram cars also had wheel bra%es and trac% slipper bra%es which could stop the tram should the electric bra%ing s$stems fail. 0n several cases the tram car motors were shunt wound instead of series wound# and the s$stems on the Cr$stal Palace line utili=ed series¶llel controllers.clarification
needed G7G
Following a serious accident at
*awtenstall# an embargo was placed on this form of traction in 1911; the regenerative bra%ing s$stem was reintroduced 2 $ears later .6G *egenerative bra%ing has been in etensive use on railwa$s for man$ decades. ,he +a%u&,bilisi& +atumi railwa$ (,ranscaucasus *ailwa$ or
0n :candinavia the Kiruna to !arvi% electrified railwa$ carries iron ore on the steepl$&graded
route from the mines in Kiruna# in the north of :weden# down to the port of !arvi% in !orwa$ to this da$. ,he rail cars are full of thousands of tons of iron ore on the wa$ down to !arvi%# and
these trains generate large amounts of electricit$ b$ regenerative bra%ing# with a maimum recuperative bra%ing force of 742 %!. From *i%sgrOnsen on the national border to the Port of !arvi%# the trains9G use onl$ a fifth of the power the$ regenerate. ,he regenerated energ$ is sufficient to power the empt$ trains bac% up to the national border.12G An$ ecess energ$ from the railwa$ is pumped into the power grid to suppl$ homes and businesses in the region# and the railwa$ is a net generator of electricit$. Blectric cars used regenerative bra%ing since the earliest eperiments# but this was often a comple affair where the driver had to flip switches between various operational modes in order to use it. ,he +a%er Blectric *unabout and the @wen 5agnetic were earl$ eamples# which used man$ switches and modes controlled b$ an epensive Dblac% boD or Ddrum switchD as part of their electrical s$stem.11G1G ,hese# li%e the Krieger design# could onl$ practicall$ be used on downhill portions of a trip# and had to be manuall$ engaged. 0mprovements in electronics allowed this process to be full$ automated# starting with 1967Hs A5C Amitron eperimental electric car. esigned b$
the wheels. ,his reversal actuall$ ma%es it perform li%e a power generator or d$namo that produces electrical energ$. ,he electricit$ developed is routed towards the carHs storage batteries to recharge them. At higher speeds# regenerative bra%es still re"uire the assistance of traditional bra%e s$stems to be applied as a bac%up. ,he efficienc$ of regenerative bra%ing s$stems in use toda$ has improved significantl$. :ome eisting s$stems are able to capture and store as much as 72 percent of the energ$ that would otherwise have been lost. ,his recapturing and storing of electrical energ$ ma $ be li%ened to Dtric%leD charging of the batteries. ,his is because most of the time# the electric motor runs in tor"ue producing mode to drive the vehicle. ,he recommended batter$ charging method still has to be performed to charge the batteries full$# although regenerative bra%ing does translate to an increase in vehicle range.
#AI* SA#E B+A,ES The term fail-safe brake refers to a type of brake that engages to prevent shaft rotation when electrical power is removed for any reason. When power is restored, the brake releases and stays in the off position. Like all friction clutches and brakes, fail-safe brakes generate torque through friction surfaces that are clamped together. The source of the clamping force distinguishes the two basic types permanent magnet and spring-set. !n general, permanent-magnet brakes are used in applications that require frequent onoff cycling and consistent performance, whereas spring-set brakes are better suited for static holding applications and low-cycle dynamic operation.
Permanent-magnet brakes "ail-safe brakes of the permanent magnet type work essentially the same as electric brakes e#cept that the magnets generate a flu# that clamps the friction surfaces together, "igure $. %haped like a horseshoe, the magnet assembly directs magnetic flu# through inner and outer &'orth and %outh( poles, which attracts the armature. When the armature contacts the magnet assembly, it completes a magnetic circuit to engage the brake and provide stopping torque through the shaft to the driven equipment. "riction material between the inner and outer poles contacts the armature and reduces wear. When power is applied to the brake, a magnetic coil in the magnet assembly generates an equal, but opposite, magnetic force that counteracts the permanent- magnet flu# and releases the brake. %electing the correct power supply for the coil is critical to the operation of permanentmagnet, fail-safe brakes. Though release voltages may be specified at )* or + dc, these values vary slightly from one brake to another. Therefore, the power supply must have an adustable voltage output so the voltage can be set at a value that causes the coil flu# to cancel the permanent-magnet flu# and cleanly release the brake.
To ensure a strong attractive force between armature and magnet, the friction surfaces must be clean and burnished. Burnishing is a process in which the manufacturer runs the unit to breakin the friction surfaces, thereby ma#imi/ing friction and torque. !n dynamic applications, the friction surfaces slip upon each engagement &which continues the burnishing effect(, keeping them free from corrosion and debris. This slippage maintains correct alignment and full contact of the friction surfaces. 0ermanent-magnet brakes are usually equipped with some type of mechanism to automatically compensate for friction surface wear. This mechanism maintains a constant air gap between the armature and magnet assembly to ensure consistent stopping time throughout the brake life. "rom a cycling standpoint, permanent magnet brakes provide more consistent performance torque and stopping times remain the same throughout the life of the brake. 1nd they are well suited to applications where cycling rates range from about 2 to $* cpm or higher.
Spring-set brakes The three basic categories of spring-set brakes are electrically released, hydraulically released, and pneumatically released. 1ll types use springs to provide clamping pressure when power is removed. Though electrically released brakes are e#plained here in detail, hydraulically and pneumatically released brakes function in a similar way e#cept that a hydraulic or pneumatic cylinder is used to release the brake rather than an electrical device. 1lso, hydraulic and pneumatic units have the same basic si/ing and selection criteria as electrical types. 3lectrically released spring-set brakes use a system of springs and a magnetic coil, "igure +. Without electrical power, the springs clamp the rotor &with attached friction pads( between a stationary end plate and a non-rotating armature, generating brake torque. When power is applied to the coil, magnetic force pulls the armature toward the coil, compressing the springs and releasing the brake. 0ower supplies for electrically released spring-set brakes are less critical to their operation than those for permanent- magnet brakes. 1 simple, fi#ed-voltage power supply is all that is required to release the brake.
%pring-set brakes are well suited to static holding applications where a servo or step motor brings the load down to speed. When the motor is turned off, the brake holds the load in position. This type of brake can also handle occasional emergency stops. Continue on page 2 These spring-set units do not normally include a wear adustment mechanism. When used in dynamic applications, however, you can compensate for wear by manually repositioning the friction components. Because spring-set brakes are applied mechanically, it4s easy to install a manual release on them. By contrast, separating the armature and magnet in a permanent- magnet brake can require considerable force. 5ere, a separate power supply may be required to release the brake.
Application and selection 0arameters involved in designing a fail-safe system include the type of engagement, accel6decel time, motor torque, rate of energy dissipation, and mounting configuration. Engagement type. 1 primary selection parameter is the type of engagement, either static or dynamic. 0ermanent-magnet, fail-safe brakes are best for demanding dynamic applications because they automatically adust for wear. Accel/decel. 7ynamic applications may require a specific deceleration time. This can be accomplished by si/ing the brake based on system speed and the inertia reflected to the brake. 1 more common way is to si/e the brake based on the motor torque.
Motor torque. 8onsider the k factor of the motor in calculating torque, not ust t he steadystate $**9 torque rating. "or short periods of time, motors can draw e#tra current to provide more torque than the $**9 rating. This is particularly important in si/ing brakes for lifts and inclined conveyors. The brake should be able to stop anything that the motor can lift. To achieve this capability, make sure the dynamic brake torque at operating speed e#ceeds the torque developed by the motor. !f there is enough dynamic torque to stall the motor, the brake can stop any load that the motor can lift. To determine the required brake si/e, first calculate the motor4s dynamic torque capability, using the formula:
where: T ; Torque, lb-ft P ;
2.
where: E ; 3nergy per cycle, ft-lb WR+ ; !nertia , lb-ft + N ; %peed, rpm "or repetitive cycles, the rate at which energy is put into the brake is: E i ; E ? f where: E i ; @ate of energy input, ft-lb6min f ; 8ycling frequency, cycles6min Continue on page 3 1 brake4s ability to dissipate heat increases with speed. Because the brake spends part of each cycle at /ero speed and part at operating speed, its average energy dissipation capability is:
where: E o ; 1verage rate at which brake dissipates energy &energy output(, ft-lb6min t 1 ; Time at /ero speed, sec t 2 ; Time at operating speed, sec E 1 ; 3nergy rate at /ero speed &from manufacturer4s heat dissipation curve(, ft-lb6min E 2 ; 3nergy rate at operating speed &from manufacturer4s heat dissipation curve(, ft-lb6min The average heat dissipation rate of the brake & E o( must equal or e#ceed the energy rate created by stopping the load & E i (. 3#ceeding the heat dissipation rating of the brake could cause inconsistent performance and premature brake failure. 5ori/ontal conveyors often use failsafe brakes ust for convenience: when the motor stops, the brake maintains the conveyor position. 5ere, an oversi/e brake may stop the conveyor too fast, causing the load to slip or tip over.
Mounting coniguration. %everal brake mounting configurations are available including shaft mounted, flange mounted, and modular. 1 shaft-mounted brake attaches to a shaft via a tapered bushing, and the torque is taken up by a torque arm, "igure $. With a flangemounted brake, the magnet is attached to a stationary part of the driven machine. There are two basic modular types: one fits on the motor shaft and the other fits on a special motor brake shaft. 1 brake module that attaches to the motor shaft can be used to convert a '3<1 8face motor into a brake motor.
Ensuring saety %imply installing a fail-safe brake in a drive system does not guarantee that the system will fail safely. 8reating a safe system requires good design practices and coordination of redundant components to eliminate potential ha/ards to personnel and equipment in the event of power loss. 7esigners should refer to appropriate !%A and 1'%! standards for guidelines regarding application and safety factors that need to be considered. The particular standards will depend on the type of equipment and its intended use. 1dditionally, periodic inspection, testing, and verification of system performance must be implemented to ensure correct operation of fail-safe systems.
Fail-Safe Brakes
More properly called "split circuit" or "dual braking" system, in which the hydraulic system is in two isolated parts so that a rupture in any one item in the system will not render the whole of the braking system inoperative. n the simplest type the bore of the master cylinder is divided into two compartments by a floating piston. ne compartment serving the front brakes and the other the rear brakes
Antis!id System "#eration
When bra%e force is applied to a vehicle wheel that is in normal contact with the pavement# the rubber of the tire begins to stretch in response to friction heating and the force applied to the tire& pavement interface. ,his has the effect of ma%ing the tire circumference significantl$ larger than it is without the bra%es applied.
When bra%e force is applied# the angular velocit$ of the bra%ed wheel drops b$ several percent. Barl$ researchers thought that this slowing down was the result of the tire slipping against the pavement and coined the term Dslip velocit$D to epress the difference between the circumferential speed of the bra%ed and un&bra%ed wheels. 0f the level of bra%ing is increased until the co&efficient of friction# mu# can no longer support the force being applied to the rubber# then true slip begins and the available stopping force begins to diminish.
@peration at the pea% of the mu&slip curve gives the highest bra%ing efficienc$. *esearch suggests that a small level of true slip ma$ increase mu and that the pea% of the curve actuall$ occurs after true slip has begun. @peration ?ust be$ond the pea% of the curve results in increased tire wear and if the bra%e force is further increased# a s%id develops that ma$ loc% the wheel and blow the tire if unchec%ed. Aircraft tires can blow in as little as 322 milliseconds at high speeds if the wheel is loc%ed.
5odern '$dro&Aire bra%e control s$stems wor% b$ measuring the speed of the wheel to determine slip and developing a correction signal to ad?ust bra%e pressure to %eep the tire operating at the pea% of the mu&slip curve. A rotar$ transducer# which is usuall$ mounted in the aircraft ale# measures wheelspeed and provides a signal to an electronic bra%e s$stem control unit (+:C). ,he control unit derives where the tire is operating on the mu&slip curve for the prevailing runwa$ conditions and sends a correction signal to the antis%id valve to reduce applied bra%e pressure.
